Offset casters cause problems

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I have a robot that looks like the Ubiquity: differential drive, two powered wheels and two casters offeet casters. I like the stability it gives and that it’s more rugged than a ballbearing-wheel.

I works mostly correctly. However not really. If the robot is going straight the casters are aligned and it goes straight. Now if I request a turn, especially a sharp turn (going very slowly) the casters take a moment or three to re-aligh around the turn. Similarly if I stop the robot while it was in a sharp turn and then start again asking for forward, initially the casters make it turn more.

As a result it seems like accurate positioning of the robot is very bad. If it needs to drive forward 10 cmd and it had been turning there’s no way to do it. This makes SLAM navigation really struggle.

Does the Ubiquity have this problem? How does it solve it? Any suggestions? Is it the case that if I want any kind of accurate positioning this configuration just won’t work? What are alternatives? I need this robot to work outdoors so the little ballbearing casters are not really an option.

Thanks for your insights!

Pito Salas
Faculty, Computer Science
Brandeis University

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Ah yeah I know what you’re talking about, I’ve recently been working on a scaled down version of it and it has those same problems that you mention.

Interestingly enough I’ve never seen them happen much on the actual Magni, which I suspect is due to the mass. The metal frame and lead acids give the drive wheels immense grip, so even if the rear casters are out of position they’re dragged to comply with the required direction. They do nudge the robot slightly in that extreme case when they have to be turned around completely, but it’s really minor for the most part and I haven’t seen it affect odometry much.

I think part of it is also that the casters are quite loaded up, so they have too much grip to be dragged sideways for long and quickly turn around. I’d suggest adding more weight to your robot and see if the handling improves. I suppose it’s also worth noting that the casters need to be installed exactly paralel to the floor otherwise you’ll get that wiggling that shopping carts get.

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Thanks for the info. I will do some experimenting on this when I next see my robot :slight_smile:

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